Metal-modified silica adsorbents for removal of humic substances in water

As novel adsorbents for humic substances, Fe-, Mg-, and Ca-modified silica gels SiO2-Fe, SiO2-Mg, and SiO2-Ca were prepared, and their adsorbabilities to humic, and fulvic acids were evaluated in water at 25degreesC for 20 h. Among these adsorbents, SiO2-Fe indicated the highest adsorbability, in which removing humic substances in water was accomplished to 80-97%. By calcination at 600degreesC and washing with water, adsorbabilities of the silica adsorbents deteriorated except for the case of calcination of SiO2-Ca. This is due to changing metal modification structures and losing chlorine, judging from elemental analysis, TG-DTA, and XPS. Especially, the modifier Fe was tightly fixed on the silica surface of SiO2-Fe, since the metal content was almost constant even after the calcination and water washing unlike the other adsorbents. Therefore, we found that SiO2-Fe is the most useful adsorbent among the silica adsorbents and that its modification structure is composed of Si-O-Fe, Fe-Cl, and Fe-OH. In addition, the adsorption mechanism is explained by an interaction between Fe and humic substance molecule having carboxylate and phenolate groups, accompanied with anion exchange of chloride ion Cl- for the humic substance molecule via the silica pores. SiO2-Fe may be applied to an adsorbent alternative for charcoal in water purification plants, and the used SiO2-Fe may be further reused as a fertilizer since humic substances have plant-glowing ability and silica strengthens plant parts. (C) 2004 Elsevier Inc. All rights reserved.